Discrete light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. However, the high cost of LEDs and associated heat-sinking and thermal-management systems have limited the widespread utilization of LEDs, particularly in general lighting applications.
The high cost of LED-based lighting systems has several contributors. LEDs are typically encased in a package and multiple packaged LEDs are used in each lighting system to achieve the required light intensity. In order to reduce costs, LED manufacturers have developed high-power LEDs, which can emit relatively higher light intensities by operating at relatively higher currents. While reducing the package count, these LEDs require relatively higher-cost packages to accommodate the higher current levels and to manage the significantly higher heat levels that result. The higher heat loads and currents, in turn, require more expensive thermal-management and heat-sinking systems that also add to the cost as well as to size of the system. Higher operating temperatures also lead to shorter lifetimes, degradation in luminous output, and reduced reliability.
Conventional LED thermal-management and heat-sinking systems have been devised to improve the heat transfer from the light-emitting junction in the LED to external heat-dissipating elements. Heat-dissipating elements typically include thermal slugs in the package, ceramic or metal submounts, large metal or ceramic heat sinks, and the like. Another example of a conventional heat-dissipating element is a high thermal conductivity wiring board, such as a metal core printed circuit board. Thermally conductive underfills may also be used to aid in transfer of heat from the package to the wiring board.
In conventional systems, the electrical traces may be utilized as heat-dissipating elements; in such systems, the LED typically has a pair of electrical contacts in electrical and thermal engagement with the traces, which are typically composed of a metallic material. Heat generated by the LED flows through its electrical contacts into the traces. Heat may also flow out the body of the LED, and be dissipated through an underfill that has a relatively high thermal conductivity. Typical underfills include thermally conductive adhesives or pastes. Typically, the traces and the underfill are thermally coupled to a high thermal conductivity substrate, such as AlN, a metal core printed circuit board, or other high thermal conductivity material. Because the substrate has high thermal conductivity there is a relatively large area—approximately the entire chip area—through which heat may flow from the chip to the substrate. The high thermal conductivity substrate dissipates heat transferred to it and this substrate is therefore an important element in thermal management of high-power LED-based lighting systems.
An alternative approach to conventional high-power LED-based lighting systems is the use of an array of relatively small LEDs mounted on a low cost plastic base substrate and driven at relatively low current. Examples of such systems are disclosed in U.S. Pat. No. 8,384,121, filed Jun. 29, 2011 (the '121 patent), U.S. Patent Application Publication No. 2014/0062318, filed Mar. 13, 2013, and U.S. Patent Application Publication No. 2014/0062316, filed Aug. 19, 2013, the entire disclosure of each of which is incorporated herein by reference. Such systems typically feature conductive traces, formed over a plastic substrate, and which are used to interconnect and provide power to an LED through its electrical contacts. When the LEDs on the plastic substrate are operated at low drive currents, no additional thermal management may be required, because the relatively small amount of heat generated by each LED is distributed over the entire array area. Such heat distribution may be through the conductive traces, which typically have a relatively larger thermal conductivity than the plastic substrate. Thus, while these systems typically have significantly lower cost because there is no need for additional heat sinking or thermal management, they may be limited to relatively low drive currents.
In view of the foregoing, a need exists for systems and techniques enabling the design and manufacture of LED-based lighting systems, as well as corresponding thermal-management techniques, capable of supporting relatively high LED drive currents at low cost.